The present invention relates to a method of repairing metallic components, and in particular to a method of repairing superalloy turbine blades and nozzles.
Over the years, superalloy materials have been developed to provide mechanical strength to turbine blades (or xe2x80x9cbucketsxe2x80x9d) and nozzles (or xe2x80x9cvanesxe2x80x9d) operating at high temperatures. Most modern high temperature superalloy articles such as nickel-based, precipitation strengthened superalloys are complex alloys at the cutting edge of high temperature metallurgy, and no other class of alloys can match their high temperature strength. This strength makes these alloys very useful in high-temperature high-strength requiring applications, such as turbine components.
These components are difficult and expensive to manufacture, and it is far more desirable to repair a damaged component than to replace one. As a result, a variety of repair methods have been developed, such as conventional fusion welding, plasma thermal metal spraying, brazing, etc. These processes are most suitable for providing relatively thin coatings of weld material. Narrow-gap brazing techniques have been plagued by joint contamination that results in incomplete bonding, even when elaborate thermochemical cleaning processes precede the brazing operation. Narrow gap brazing also lacks the ability to restore damaged or missing areas on a superalloy component or turbine blade. Joints formed using wide gap brazing methods can be difficult to set-up and porosity in the deposited filler material continues to be a concern.
Traditional weld repair methods that are capable of providing thicker coatings, such as gas tungsten arc welding (GTAW) and plasma transferred arc welding (PTAW), have met with only limited success. These traditional methods have been unsatisfactory because the quantities of certain precipitate-forming elements (mainly aluminum and titanium) that are added specifically to superalloys for high temperature strength cause traditional methods to produce poor welds using superalloy weld fillers. Although GTAW and PTAW are the methods most commonly used in turbine blade repair today they use lower strength weld fillers. Thus, their current use is limited to certain blade surfaces that experience very low stress and to other components that are made with other materials. Turbine nozzles, for example, are currently made with cobalt-based superalloys that lend themselves to repair using current welding or brazing methods.
More specifically, weld quality is poor because the elements added for high temperature strength result in welds that have a tendency to form or contain cracks. Two distinct types of cracking have been identified: (1) hot cracking and (2) strain age cracking. Hot cracking occurs in the filler metal and heat affected zone (HAZ) during welding and is typically in the form of tiny fissures, or micro-cracks, beneath the surface of the weldment. Strain age cracking occurs during post weld heat treatment, usually initiating in the HAZ and often propagating well into the adjacent base alloy. Strain age cracks are generally much longer than hot cracks, sometimes extending several inches into the base material.
Weld filler materials that have been most effective in the repair of precipitation strengthened superalloys are those that do not cause hot cracking or strain age cracking. These filler materials are simpler, solid-solution strengthened alloys, but they have significantly lower strength than the superalloys. Therefore, the use of low strength filler materials significantly limits the locations on certain components where weld repairs can be made.
For example, current industry practice for turbine blades permits welding only in areas of very low stress, and some 80 to 90 percent of blade surfaces are non-repairable. Blades with non-repairable damage are generally returned to suppliers as scrap for credit against replacement blades. The financial impact on utilities is considerable since a single air-cooled, rotating blade may cost up to thirty-five thousand dollars, and, depending upon the turbine manufacturer and model, each turbine has multiple blade rows consisting of approximately 90 to 120 blades per row. Turbine blades, however, are not the only components employing high temperature superalloys and requiring repair. Advanced turbines will employ more components made with the more sophisticated superalloys, thereby increasing the number of different superalloy components that may need weld repair. In fact, it is anticipated that future turbine nozzles will be made of nickel-based superalloys such as GTD-111.
Various studies have been conducted to evaluate methods for the repair of precipitation strengthened superalloys. These studies have included evaluations of both narrow and wide-gap brazing, gas tungsten arc welding (GTAW), plasma transferred arc welding (PTAW), and electron beam welding (EBW).
Many experts believe that low heat-high energy welding processes have the highest potential for advancing the state of the art for blade repair. The use of such processes has been shown to reduce cracking while using superalloy weld filler. Laser beam welding (LBW) and EBW are both low heat-high energy processes capable of providing small volume welds with narrow heat affected zones. The laser welding process has seen limited use in the repair of IN-738 superalloy turbine blades. When employed, laser welding has been restricted to regions of very low stress using solid solution strengthened weld filler alloys, mainly IN-625, which provide mechanical properties significantly inferior to those of the base IN-738 material. Structural weld repairs that extend into the more highly stressed regions of the blade cannot be performed currently. EBW is currently being used for the repair of gas turbine stationary nozzles, combustion components, and shaft seals where the joint geometry is relatively straight or in one plane. EBW has inherent limitations in weld path flexibility and must be performed in a vacuum chamber. Application of EBW in the repair of complex blade airfoil shapes would require significant development and is not considered practical at this time. In view of the foregoing, it would be highly desirable to provide an improved technique for repairing metallic parts, such as superalloy turbine blades or other superalloy components.
According to the invention there is provided a method of repairing a metallic component, such as a superalloy turbine blade or nozzle. The component is prepared by stripping the protective coatings from it. The method of repairing the metallic component comprises subjecting the metallic component to a first hot isostatic processing operation, welding the metallic component, and exposing the metallic component to a second hot isostatic processing operation. In addition, when welding the metallic component, weld fillers may be added to the weld area. The component is finally prepared for re-entry into service.
One of the advantages of the technique of the invention is that the use of precipitation strengthened filler superalloys more closely matches the mechanical properties of the base alloy. Another advantage of the invention is that the use of high energy-low heat methods, such as EBW, as the welding heat source, as opposed to conventional arc welding processes, produces smaller heat affected zones and reduces the stress field due to the lower quantity of heat introduced in the weld zone. A further advantage of the invention is that the introduction of a dual hot isostatic process, which brackets the welding application, preconditions the substrate for welding and reduces any micro-cracking inherent with the superalloy blades after welding.
This repair methodology provides a means to extend the current limits of repair to the more highly stressed areas of the component, or to repair components made of superalloys in general. Thus, the invention also includes components repaired according to the methods of the invention, or a metallic component repaired according to a method comprising subjecting a metallic component to a first hot isostatic processing operation, welding the metallic component, and exposing the metallic component to a second hot isostatic processing operation.